This invention relates to devices for storing and transporting semiconductor wafers and more particularly to an improved wafer packaging system for clean packaging and damage-free transporting of semiconductor wafers, especially such a system for long-term storage of such wafers.
Electronic devices such as transistors and integrated circuits are made from semiconductor structures, primarily of silicon. Such structures are produced by the epitaxial deposition of silicon or similar materials over a substrate wafer formed of the same material. Generally, the wafers involved must be formed of single crystal silicon with precisely controlled concentrations of dopant impurities.
Typically, the semiconductor wafers have a highly polished surface which must remain free from contact with any foreign surface whatsoever during storage and transporting thereof. It has generally been found that when this polished surface contacts any foreign surface, the surface tends to become contaminated and marred. These slight marrings of the surface and the impurities often materially interfere with effective further processing of the wafers such as growth of epitaxial layers or fabrication of semiconductor devices. In the past, transporting of the wafers between steps in processing thereof has often caused serious problems since it was not always possible to maintain the wafers in a substantially clean atmospheric environment. Furthermore, accidental contact with the surface of such wafers often has occured through inadvertence which, in effect, has made susbsequent processing steps futile and destroyed the value of the wafers.
Several packaging schemes have been employed in past years in an attempt to avoid scratching and contamination of wafers. An early arrangement for silicon wafers was to stack wafers like coins in vials with paper inserts between each slice. This resulted in paper lint on the wafers and did not avoid surface scratches, which often occurred upon unpacking.
Another packaging method involved the use of individual plastic boxes for each wafer, the box having layers of foam rubber placed below and above the wafer. However, this type of package often caused contamination of wafers from shredded foam, was relatively expensive and was time consuming to use.
Glassine bags were employed for a time but it was difficult to insert and remove the wafers from the bags. In addition, the bags were often not entirely contaminant free. Moreover, they did not protect wafers from breakage.
A substantial improvement was represented by the introduction of "dimple tray" wafer packages of molded synthetic resin material and having a flat surface with recessed, or "dimples," each adapted for containing a wafer. Such plastic trays have become widely used because of their many advantages yet they do not ensure completely against contamination of wafers by plastic slivers, mold release agents, solvents and other vaporous contaminants. In addition, while they provide some protection, they do not completely protect against wafer breakage under some conditions. Also wafers must be manually transcribed from such packages for further processing and it is increasingly desirable to avoid such manual steps.
Another problem associated with storage of such wafers is that, over a period of time, the wafers tend to become oxidized. This produces an oxide layer which can interfere with subsequent processing of the wafers into electronic structures and, since it cannot be economically removed without special treatment, may render the wafers totally unfit for use. Such oxidation occurs whenever the wafers are stored in an atmospheric environment and may be affected by humidity or other conditions beyond the control of the wafer manufacturer or purchaser. In any event, oxidation such as this seriously reduces the shelf life of the semiconductor wafers.
In addition to the problems of contamination and oxidation of semiconductor wafers, their shipment to customers poses the problem of maintaining their structural integrity during adverse shipment conditions. For example, droppage or rough handling in shipment resulting in breakage of the semiconductor wafers has been a serious problem. The breakage of just one semiconductor wafer in a package may also cause the loss of several other wafers because small sharp fragments of a broken wafer may settle as dust on other wafers in the package or even severely scratch their surfaces. Hence, other wafers in the package may either require costly cleaning before usage or may be rendered useless because of scratches.